1. Field of the Invention
This invention concerns a semiconductor device such as a bipolar transistor (hereinafter simply referred to as BT) which is manufactured by using a multilayer structure material formed through the epitaxy of semiconductor materials of different properties and which is useful as an ultrahigh frequency and ultrahigh speed transistor and, particularly a heterojunction bipolar transistor (hereinafter simply referred as HBT) a semiconductor laser and a light emitting diode, as well as a method of manufacturing same.
2. Description of the Prior Art
High speed operation factors f.sub.t and f.sub.m for BT are represented as: ##EQU1## where .tau.e (emitter depletion layer transit time)=re(Cbc+Ceb+Cpb),
.tau.b (base trainsit time)=Wb.sup.2 /nDb, PA1 .tau.c (collector depletion layer transit time)=Wc/2Vs, .tau.cc (collector depletion layer charging time)=(Ree+Rc).(Cbc+Cpc), PA1 Rb: base resistance, PA1 Cbc: base-collector capacitance, PA1 Ceb: base-emitter capacitance, PA1 Cpb: base layer parasitic capacitance, PA1 Cpc: collector layer parasitic capacitance, PA1 Wb: base layer thickness, PA1 Db: base layer diffusion coefficient, PA1 Wc: collector depletion layer thickness, PA1 Vs: collector transit speed, PA1 Ree: emitter contact resistance, and PA1 Rc: collector resistance.
As can be seen from the formulas, it is important in a BT to decrease the junction capacitance such as the Cbc and Ceb, the base layer thickness, the ohmic contact resistance and the base resistance in order to increase f.sub.t and f.sub.m. Above all, it is important to decrease the base resistance Rb and Cbc in order to increase the f.sub.m. In view of the above, attempts have been made for reducing the size of transistors.
In an HBT using a semiconductor material as an emitter having a band gap larger than that of the base, since the leaks of positive holes from the base to the emitter is suppressed (in the case of npn type), the base can be doped to a higher extent, while the emitter and the collector can be doped to a lower extent contrary to an ordinary BT. Since this can reduce the base resistance Rb which is important for increasing the operation speed and the frequency of the transistor, it is advantageous for the increase of the factor f.sub.m. Further, in the usual bipolar transistor, Cbc and Ceb are represented as a product of the factors Ceb (n, h), Cbc (n, h) of the junction capacitance depending on the doping and the junction areas Aeb, Abc. Since the emitter and the collector are doped at a low concentration while the base is doped at a high concentration in the heterojunction bipolar transistor, Ceb (n, h) and Cbc (n, h) are dependent only on the doping of the emitter and the collector and Ceb and Cbc are represented as: EQU Ceb.varies..sqroot.Ne.multidot.Aeb Cbc.varies..sqroot.Nc.multidot.Abc
Accordingly, since Ceb and Cbc are made smaller in an HBT by reason of the hetero-structure as compared with the usual BT, .tau.e and .tau.cc are made smaller to provide an advantage for the increase of f.sub.t. Further, since the Cbc is made smaller, it provides an advantage in that f.sub.m is increased coupled with a decrease in Rb. For making the HBT operation speed higher, it is important to optimize the device structure in addition to the optimization of the hetero-structure itself. A normal structure in which the emitter is situated above and an inverted type in which the collector is situated above are fundamentally possible in the usual BT as the well as HBT (H. Kroemer: "Hetero-structure Bipolar Transistors and Integrated Circuits" Proc. IEEE, vol. 70, p. 13, 1982). In the normal type, it is advantageous to decrease the size of the emitter for making the Ceb smaller. In addition, if the collector can be made smaller by making the periphery of the collector semi-insulating, a remarkable increase in the f.sub.t, f.sub.m is possible since the Cbc can be made smaller (P. M. Asbeck, et al. "GaAs/(Ga,Al)As Heterojunction Bipolar Transistors with Buried Oxygen Inplanted Isolation Layers", IEEE, EDL-5, p. 311 (1984)). Further, in a inverted structure, since the collector is situated above, it is essentially advantageous to reduce the size of the collector for reducing the Cbc. Further, since the Ceb is reduced if the periphery of the emitter can be semi-insulated, f.sub.t and, accordingly, f.sub.m can be increased. It has been attempted, for such an objective, to fabricate semiconductor materials of different properties into a multi-layer structure suitable for the manufacture of BT through an epitaxy and then making the periphery of the collector at a specific portion inside of the multilayer structure semi-insulating by ion inplantation from above (P. M. Asbeck, et al. "GaAs/(Ga,Al)As Heterojunction Bipolar Transistors with Burried Oxygen Inplanted Isolation Layers", IEEE, EDL-5, p. 311 (1984)).
However, introduction of impurities into the specific portion inside of the multilayer structure from above by means of ion implantation or diffusion is not desirable as it produces undesired effects on the transistor characteristics. This is because the introduced impurities form a distribution in the direction of the depth and the impurities are distributed above the specific portion requiring the introduction of the impurities and the crystals are inevitably injured due to the great amount of impurities passed therethrough.